The long-term goals of this project are to elucidate the virological and immunological mechanisms of HIV-1induced thymus destruction, and to identify novel viral and host targets for interventions in AIDS therapy. The thymus serves as an early site for HIV-1 replication and its destruction by HIV-1 correlates with accelerated disease progression. Using SCID-hu Thy/Liv mice (human fetal thymus and fetal liver fragments engrafted in SCID mice) and human fetal thymus organ culture (HF-TOC.) models, we have recently shown that, after early reversion in vivo, an attenuated HIV- 1 env gene can support HIV- 1 replication in the thymus without pathogenicity, and the HIV-1 nef gene can function as a "pathogenic" factor (i.e., not required for replication) in the recombinant HIV- 1 in the human thymus. The reverted Env protein interacts with CD4 and CXCR4 with similar affinity, but may trigger signaling events to support HIV-1 replication in the thymus. We have also identified HIV isolates, and derived their env genes, with "thymus-tropism" for replication and/or pathogenicity from patients whose CD4 and CD8 T cells are depleted rapidly after infection. The central hypothesis is that HIV-1 infection and viral pathogenic factors alter the thymus organ (via viral and host paracrine factors) to deplete both infected and uninfected thymocytes. I will apply viral genetics, immunological and cell biological techniques in the SCID-hu Thy/Liv and HF-TOC models to elucidate the mechanisms by which the HIV-1 encoded pathogenic factors (nef and env) mediate HIV replication and pathogenesis in the thymus. How HIV replicates and leads to destruction of the thymus, and how to restore its function, are among the most important questions of HIV-1 pathogenesis and therapy in both pediatric and adult patients. First, I propose to define and characterize the nef pathogenic activity that induces thymus destruction and whether the pathogenic activity of nef is correlated with HIV-1 disease progression and the mechanisms of nef-mediated thymocyte depletion (Aim 1). Second, I plan to elucidate the mechanisms of LW-like env-mediated HIV-1replication in the thymus (Aim 2). Third, I propose to investigate the pathogenic activity of HIV-1 env in the thymus (Aim 3). If mechanisms of HIV-1 viral factors (nef and env) determining replication and thymocyte depletion in the thymus are elucidated, significant insights will be gained in defining novel viral or host targets for therapy and in restoring thymus functions in AIDS therapy, as well as in understanding HIV-1pathogenesis in the thymus.